In, for instance, the analog transmission of TV-signals over optical fibres, it is highly desirable to be able to modulate a transmitted carrier wave linearly. Non-linear modulation results in intermodulation distortion which disturbs neighbouring channels. Carrier waves having for instance the frequencies of 50 Mhz, 100 Mhz and 150 Mhz can be transmitted on optical fibres. When the modulator transmission function is non-linear, the two first-mentioned carrier wave frequencies are liable to be added together and therewith disturb the frequency of 150 Mhz.
One typical method of modulating carrier waves in the aforesaid application is to use a laser diode that has a constant light power and whose outgoing lightwave is modulated with an external modulator. The type of modulator used is often a so-called Mach-Zehnder modulator, which basically has a sinusoidal transmission function. This transmission function can be linearized, for instance, in the manner disclosed in an article in SPIE, Vol. 1102, Optical Technology for Microwave Applications IV (1989), pp. 20-29, by J. J. Pan: "High Dynamic Range Microwave Electro-Optic Modulators". The article describes, with reference to its FIG. 3, a modulator that has two parallel-coupled electrooptic Mach-Zehnder modulators. An incoming lightwave is divided between the modulators and is modulated in one of the Mach-Zehnder modulators by an electric microwave signal of desired fundamental frequency. Because the modulator is non-linear, harmonics of the fundamental frequency appear in the modulated light signal. Compensation is made for an undesirable contribution from the first occurring harmonic with three times the fundamental frequency. This is achieved by modulating the incoming lightwave in the other of said Mach-Zehnder modulators, and the lightwaves from the two modulators are mutually superimposed at the modulator outlet. The undesirable contribution from the first harmonic can be totally compensated for by suitable choice, among other things, of the modulation voltages applied to the two modulators.
A linearized Bragg-modulator is described in an article by P. R. Ashley and W. S. C. Chang: "Linearization technique for a guided wave electrooptic Bragg modulator", Proceedings IGWO '86, poster paper THCC 12. This modulator has two parallel-coupled Bragg-elements and its transmission function is compensated for the first occurring harmonic. This compensation is effected in a manner which corresponds to the manner in which compensation is effected in the parallel-coupled Mach-Zehnder modulators in the aforesaid article by J. J. Pan.
The drawback with the aforesaid modulators is that only the first occurring harmonic is compensated for or counteracted. Compensation for further overtones can be effected by coupling several modulator elements in parallel. Such modulators, however, are complicated and it is found that only small improvements are obtained. In some applications, a totally non-compensated Mach-Zehnder modulator will result in lower intermodulation distortion than a modulator in which the first occurring overtone is compensated for in the aforesaid manner.
A method for compensating the first occurring harmonic by a Mach-Zehnder modulator is described in an article in IEEE Journal on Selected Areas in Communications, Vol. 8, No. 7, pp. 1377-1381, Sep. 1990, by S. K. Korotky and R. M. de Ridder: "Dual Parallel Modulation Schemes for Low-Distortion Analog Optical Transmission". According to this article, a third order intermodulation distortion is suppressed.
The Swedish Patent Application No. 9003158-4 corresponding to U.S. Pat. No. 5,161,206 of the present applicant considers the radius of curvature of the transmission function of a modulator. The output signal of a non-linear sub-modulator is compensated to a linearized transmission function, by superimposing on said output signal an output signal from at least one further non-linear sub-modulator. Conventionally, it is endeavoured to obtain a steep average slope of the linearized transmission function within a suitably selected interval of the modulator control signal. This results in good modulation of the carrier wave and a control signal of reasonable amplitude. The radius of curvature of the linearized transmission function will have the greatest possible value within this control signal interval for an optimally designed modulator.
The aforesaid modulators have very little distortion in the case of small modulation depths, although the distortion increases greatly with the modulation depth. When specifying the performance of a modulator, it is very usual to determine a constant, highest distortion level. A problem then resides in providing a modulator of great modulation depth which will keep distortion beneath the desired highest level. No advantage is gained when the modulator distortion lies far beneath the specified highest distortion level at small modulation depths.